In the drilling of well bores, the standard practice is to mount the cutting bit at the lower end of a drill string; then, when the bit needs replacement to remove the drill string. This is time consuming and expensive particularly in the drilling of deep holes. Also it requires extensive drilling in open hole. Expansive bits have been developed to undercut the area underlying the well casing; however, this has had limited application particularly in deep wells as the casing remains fixed and becomes locked in place. Also, to replace such bits the drill string must be removed. In addition, attempts have been made to provide expansive bits which can be lowered and removed by wireline through the bore of the drill string itself. Insofar as known, this type of expansive bit has not proven satisfactory.